Laminar barrier inerting for leading and/or trailing shield in welding application

ABSTRACT

The present invention relates to processes and apparatus for welding or joining metal parts which comprise moving a welding torch in a direction along a joint between said parts, causing a molten pool of metal with said welding torch along the joint and directing a laminar flow of inert fluid toward a freshly welded area behind said welding torch in the form of a trailing shield whereby the freshly welded area is protected from the surrounding air during its solidification. The laminar flow or inert fluid may also be directed along the joint ahead of the welding torch in the form of a leading shield such that turbulence in and around the joint caused by a high welding speed is reduced, thereby preventing oxygen entrainment in said molten pool of metal. The laminar flow of inert fluid is usually provided through at least one diffuser having at least one porous diffusing surface which is generally provided with a screen and/or a spatter shield.

BACKGROUND OF THE INVENTION

The invention relates in general to processes and apparatus forproviding a laminar flow of inert fluid effectively around a molten andcooling weld, puddle and in particular to processes and apparatus forproviding a laminar flow of inert fluid in the form of a leading and/ortrailing shield to protect a molten and cooling weld puddle during thewelding operation.

Generally, welding involves application of the heat generated by awelding torch to a work piece, or to a joint between adjacent metalparts in a localized manner to effect fusion between the pieces or partsto be joined. Such a welding operation usually takes place in thepresence of an inert shielding gas to prevent contamination of theregion being treated, such as a molten weld puddle, from the surroundingair. Commonly, the inert shielding gas is introduced to a molten weldpuddle area via an annular passageway or other passageways surrounding awelding torch to protect a very limited area beneath the torch from thesurrounding air. Shielding a very limited area in this manner, however,can adversely affect, inter alia, the mechanical and corrosionresistance properties and the surface appearance of a weld. Weld areas,for example, can still be subject to air contamination or oxidationduring the welding stage when an inert gas is contaminated with thesurrounding air or during the solidification stage when an inertshielding gas is no longer present.

Due to the recognition of the importance of a shielding means inobtaining high quality weld areas, various shielding arrangements havebeen proposed. For instance, U.S. Pat. Nos. 4,166,940, 4,179,059,4,528,436 and 4,567,343 show a double shielding means wherein an initialshielding means is augmented or supplemented with a secondary shieldingmeans. Inert shielding gases are introduced in such a manner as toprovide the initial shielding means generally surrounded with thesecondary shielding means. Similarly, U.S. Pat. No. 4,839,489 shows aplurality of shielding means wherein inert gases are provided in atrailing and leading fashion to protect the freshly welded area and anarea ahead of the welding torch which is preheated due to closeproximity to the welding operation. These shields, however, may not beeffective in protecting the welded area from the surrounding air as theair migrates or aspirates into the heated region. The failure to reduceturbulence, which may result from a high welding speed, around an areato be welded or to maintain a proper flow of inert fluid, for example,may cause oxygen entrainment in the welded area or oxidation of thewelded area. The effectiveness of these shielding means can furtherdeteriorate as the flow of an inert shielding gas is hindered orhampered as a result of condensing metal vapors clogging the porousdiffusing surface of a diffuser. Moreover, molten weld materials, whichmay propel away from a weld area during the welding operation, may causedamage to a welding system involved or cause an injury to a personoperating the welding system.

It is an advantage of the present invention in providing an improvedshielding arrangement wherein the quality of the finished weld isenhanced; thus reducing the need for postweld cleaning and improving thequality of postweld plating which may be applied to the finished weld.

It is another advantage of the present invention in recognizing a bettermethod for obtaining a laminar flow of inert fluid around a molten andcooling weld puddle during the welding operation.

It is yet another advantage of the present invention in increasing awelding speed while minimizing oxidation of the bead surface, i.e. theweld area.

It is a further advantage of the present invention in mitigating oralleviating the clogging effect on the porous surface of a diffuser,thereby preventing any interference in the flow of a shielding fluid.

It is an additional advantage of the present invention in providing ameans by which the weld spatter can be prevented from damaging thewelding apparatus involved.

SUMMARY OF THE INVENTION

The present invention relates to a process for welding or joining metalparts which comprises moving a welding torch in a direction along ajoint between said parts, causing the formation of a molten pool ofmetal with said welding torch along the joint and directing a laminarflow of inert fluid toward a freshly welded area behind said weldingtorch in the form of a trailing shield whereby said freshly welded areais protected from the surrounding air during its solidification. Thelaminar flow of inert fluid may also be directed along the joint aheadof said welding torch in the form of a leading shield such thatturbulence in and around the joint caused by a high welding speed isreduced to prevent oxygen entrainment in said molten pool of metal. Thedirection of said laminar flow of inert fluid and an area covered bysaid laminar flow of inert fluid are such that no freshly welded area issusceptible to oxidation and/or such that said turbulence in and aroundthe joint is reduced to prevent oxygen entrainment in said molten poolof metal. The laminar flow of inert fluid is usually provided through atleast one diffuser having at least one porous diffusing surface which isgenerally made up of a sheet of sintered metal powder.

To this at least one diffuser, at least one screen is releasably orremovably mounted to cover its at least one porous diffusing surface sothat said at least one porous surface is protected from clogging due tometal vapor condensation and possible damage from a weld spatter duringa welding operation. The screen can be readily replaced withoutadversely affecting the diffuser, once its use is no longer feasible,i.e., once it is clogged due to the metal vapor condensation. Theapplication of at least one screen on at least one porous diffusingsurface of at least one diffuser does not adversely affect a laminarflow of inert fluid due to the type of a screen design employed.Typically, the screen employed is a fine mesh screen which may beconstructed with metals or alloys such as stainless steel.

This screen can also be designed to accommodate or fit into any shieldfluid distributing apparatus which is part of a welding device. A shieldfluid distributing apparatus for a welding device, for example, maycomprise:

(a) at least one diffuser means having at least one porous diffusingsurface; and

(b) at least one screen means removably or releasably mounted on said atleast one diffuser means to cover said at least one porous surfacethereof, said screen being shaped such that the clogging effect on saidat least one porous surface, as a result of metal vapor condensationduring the welding operation, is reduced or prevented.

In addition to the screen, at least one spatter shield can be releasablyor removably mounted on at least one diffuser of a welding device. Thespatter shield is preferably placed on the front edge of a diffuser inclose proximity of a welding torch so that any large metal globules,which may be propelling away from a weld area, are deflected, therebyprotecting a welding device involved. The spatter shield generallycomprises a strip of metal.

As used herein, "a trailing shield or trailer shield" means that as awelding torch traverses and welds sections of metal to be joined, ashielding means, such as inert fluid or inert fluid producing means, isprovided behind the welding torch to cover the freshly welded hot metalwith a protective atmosphere of said inert fluid.

As used herein, "a leading shield" means that a shielding means isplaced ahead of a welding torch so that substantial sections of metal tobe joined or welded are subject to a protective atmosphere of inertfluid. The term "substantial sections" means an area covered by inertfluid is sufficient to reduce turbulence which may result from a highwelding speed.

As used herein, "laminar" flow means that the root mean square of therandom fluctuations in the fluid layer velocity at the source or originof the fluid layer are less than about 0.1 times the average velocity ofthe fluid in its direction of flow at its source of origin and that theroot means square of the sizes of turbulent eddies in the fluid layer atits source of origin are less than 0.1 times the thickness of the layerat the source of origin of the fluid layer.

As used herein, "inert fluid" means any liquid and/or gaseous substancewhich could prevent metals from oxidizing during the welding operation.The substance generally comprises, for example, argon, nitrogen, helium,hydrogen, CO², etc. . . . or mixtures thereof. The preferred substance,however is argon or a mixture containing argon which may have an inertamount of oxidizing gases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents one embodiment of the invention which illustrates across-sectional view of a welding device having trailing and leadingshields.

FIG. 2 represents one embodiment of the invention which illustrates across-sectional view of a welding device having a trailer shield.

FIG. 3 represents one embodiment of the invention which illustrates across-sectional view of a welding device having a leading shield.

FIG. 4 represents one embodiment of the invention which illustrates across-sectional view of a diffuser having a screen mounted thereon.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to an improved method for obtaining a laminar flowof inert fluid around a molten and cooling weld puddle. By using alaminar flow with a particular diffuser arrangement, the quality of thefinished weld is improved, thereby reducing the need for postweldcleaning. The quality of postweld plating that may be applied to thefinished weld is also enhanced.

Usually, this particular diffuser arrangement is used in conjunctionwith a conventional inert shielding means which is commonly used toprotect a limited area beneath a welding torch during the weldingprocess. This additional shielding is provided with a laminar flow ofinert fluid in the form of a trailing shield to protect the solidifyingweld material from oxidation as the welding torch moves out of a weldzone and/or in the form of a leading shield to reduce turbulence in aweld zone, thus preventing oxygen entrainment in the resulting weld.

The particular diffuser arrangement becomes especially useful when it ismodified by incorporating at least one screen and/or at least onespatter shield. While the life of at least one diffuser can be extendedthrough using at least one screen, the life of a welding system can beextended through using at least one spatter shield. The extension of thelife of at least one diffuser is particularly noteworthy since currentdiffuser construction makes replacement of the porous diffusing surfaceof a diffuser difficult.

Referring to FIGS. 1, 2 and 3, the direction of travel of a weldingdevice (1) is shown by the arrow. The welding device (1) comprising agas shielded welding torch (2) travels along a joint between adjacentmetal pieces (3) to be welded to cause a molten pool of metal along thejoint. The welding torch (2) comprises an electrode (7) which isdisposed inside an elongated housing means (4) to extend axiallythereof. At least one conventional shielding means (not shown) is placedon or in this elongated housing means (4) to provide inert gas around anarea which is being welded.

The housing means (4) is connected to a trailer shield assembly (5) viaa coupling means (6). The trailer shield assembly (5) is releasably orremovably mounted on the back of the welding system (1) so that as thewelding torch (2) traverses and welds the metal pieces, it follows andcovers the freshly welded area with a protective blanket of inert fluid.The trailer shield assembly (5) comprises at least one diffuser havingat least one porous diffusing surface facing toward the freshly weldedarea and at least one spatter shield (9) comprising a strip of metal oralloys, such as stainless steel, mounted on the front edge of said atleast one diffuser in close proximity of the welding torch (2) todeflect any metal globules which maybe propelled away from a weld area.

To this welding device having a trailer shield assembly, a leadingshield assembly (10) is provided to further enhance the quality of thefinished weld. The leading shield assembly (10), however, can be usedalone with a welding torch as shown in FIG. 3. The leading shieldassembly (10) is connected to the housing means (4) via a coupling means(11) on the front of the welding device (1) so that a laminar flow ofinert fluid is provided along the joint to be welded ahead of thewelding torch (2). The leading shield assembly (10) comprises anoptional spatter shield (12) placed on the front edge of at least onediffuser having at least one porous diffusing surface which size issufficient to reduce turbulence, caused by a high welding speed, alongthe joint, thereby reducing oxygen entrainment in the resulting weld.The size of the diffusing surface is such that it extends beyond an areawhich is preheated due to the preheated area's close proximity to anarea being welded.

As shown in FIG. 4, the preferred diffuser employed in both the leadingand trailer shield assemblies is generally defined by a rectangular box(13) having sides (14 and 15), a front (16), a back (17), a top (18) anda bottom diffusing surface (19). On one of the sides, there is opening(20) for receiving inert fluid which is to be diffused through thebottom surface (19) at a flow rate cf about 35 to about 100 cfh,preferably about 60 to about 100 cfh. The bottom diffusing surface (19)is constructed with a sheet of sintered metal or alloys, such asstainless steel, and has a pore size of about 1.0 to about 40 microns.The size of the diffusing surface area is defined by at least about 1.5inches in length and at least about 1.0 inches in width, more preferablyat least about 2 or about 6 inches in length and at least about 2 inchesin width. The size of the diffusing surface area, however, varies withthe type of welding operation involved, including the reason or purposefor employing the diffuser.

The life of this diffuser can be extended when it is modified byincorporating at least one screen (21). The screen (21) is usuallymounted on the diffuser via coupling means such as studs, located on thetop of the diffuser to cover the diffusing surface (19). This externallyplaced screen prevents the diffusing surface (19) from clogging bycapturing the metal fume resulting from the welding operation. Thescreen can be made with various metals and alloys. The screen, however,is preferably made with stainless steel and has a fine mesh, that is,about 320 to about 400 mesh.

The present invention imparts various advantages in welding processes,especially in tungsten arc and gas metal arc welding operations, byproviding particular shielding arrangements. The advantages can be seenin (1) the quality of the finished weld, (2) the welding speed involvedwithout compromising the quality of the finished weld, (3) the extendedlife of a diffuser and other welding units, and (4) the application of alaminar flow of inert fluid without interrupting the welding process.

Although the process and apparatus of the invention has been describedin detail with reference to certain embodiments, those skilled in theart will recognize that there are other embodiments of the inventionwithin the spirit and scope of the claims.

What is claimed is:
 1. A process for welding or joining metal partswhich comprises moving a welding torch in a direction along a jointbetween said parts, causing the formation of a molten pool of metal withsaid welding torch along the joint and directing a laminar flow of inertfluid through at least one diffuser having a pore size of about 1 toabout 40 microns toward a freshly welded area behind said welding torchin the form of a trailing shield whereby said freshly welded area isprotected from the surrounding air during its solidification.
 2. Aprocess for welding or joining metal parts which comprises moving awelding torch along a joint between said metal parts to cause a moltenpool of metal along the joint and directing a laminar flow of inertfluid through at least one diffuser having a pore size of about 1 toabout 40 microns along a joint to be welded, which is ahead of saidwelding torch, so that turbulence in and around said joint caused by ahigh welding speed is reduced, thus preventing oxygen entrainment insaid molten pool of metal.
 3. A process according to claim 1 or 2,wherein the direction of said laminar flow of inert fluid and an areacovered by said laminar flow of inert fluid are such that no freshlywelded area is susceptible to oxidation or such that said turbulence isreduced.
 4. A process according to claim 1 or 2, wherein said laminarflow of inert fluid is derived from said at least one diffuser, which isprovided with at least one screen means to protect at least one poroussurface of said at least one diffuser from clogging due to metal vaporcondensation.
 5. A process according to claim 4, wherein said at leastone screen means is releasably or removably mounted on said at least onediffuser such that said at least one screen is readily replaceable oneit is clogged due to metal vapor condensation.
 6. A process according toclaim 4, wherein said at least one diffuser is a rectangular box havingat least one porous diffusing surface which is made up of a sheet ofsintered metal powder.
 7. A process according to claim 6, wherein saidat least one porous surface is made with stainless steel.
 8. A processaccording to claim 5, wherein said at least one screen is a fine meshscreen having about 320 to about 400 mesh, which is constructed withstainless steel.
 9. A process according to claim 5, wherein at least onespatter shield comprising at least one strip of metal is provided on thefront edge of said at least one diffuser in close proximity of saidwelding torch so that any large metal globules that are propelling awayfrom a weld area are deflected.
 10. A process according to claim 1 or 2,said area covered by said laminar flow of inert fluid is defined by atleast about 1.5 inches in length and at least 1.0 inches in width.
 11. Aprocess according to claim 1, further comprising a laminal flow of inertfluid which is directed to the joint ahead of said welding torch so thatturbulence in and around said joint caused by a high welding speed isreduced.
 12. A welding device comprising:(a) an electrode disposedwithin an elongated housing means and extending axially thereof; (b) atleast one shielding means placed on or in said elongated housing meansto provide inert fluid around an area being welded; and (c) at least onediffuser means designed to produce a laminar flow of inert fluid throughat least one porous diffusing surface comprising pores having sizes inthe range of about 1 to about 40 microns mounted on said housing means,wherein the placement of said at least one diffuser means is such thatsaid inert fluid diffusing from its porous surface is directed to shieldan area to be welded, which is ahead of said welding torch, the size ofsaid at least one diffusing surface of said diffuser means being suchthat said laminar flow of inert fluid extends beyond an area which ispreheated during a welding operation in order to reduce turbulenceresulting from a high welding speed in said area to be welded, thuspreventing oxygen entrainment.
 13. A welding device according to claim12, wherein said at least one porous diffusing surface of said at leastone diffuser means is defined by at least about 1.5 inches in length andat least about 1.0 inches in width.
 14. A welding device according toclaim 12, wherein said porous diffusing surface is constructed withstainless steel materials.
 15. A welding device according to claim 12,wherein said at least one diffuser means is provided with a screen tocover its at least one porous diffusing surface, said screen beingshaped such that the clogging effect on the porous diffusing surface, asa result of metal vapor condensation during the welding operation, isreduced or prevented.
 16. A welding device according to claim 15,wherein said screen is removably or releasably mounted on said at leastone diffuser means.
 17. A welding device according to claim 16, whereinsaid screen is a fine mesh, stainless steel screen having about 320 toabout 400 mesh.
 18. A welding device according to claim 12, wherein atleast one spatter shield comprising at least one strip of metal isprovided on the front edge of said at least one diffuser means so thatlarge metal globules, which may be propelling away from a weld duringthe welding operation, are deflected.
 19. A welding device according toclaim 12, further comprising at least one second diffuser means havingat least one porous diffusing surface which has pores having sizes inthe range of about 1 to about 40 microns and which is covered with atleast one screen, wherein the placement of said second diffuser means issuch that its surface is directed toward a freshly welded area during awelding operation and wherein the shape of said at least one screen issuch that the clogging effect on said at least one porous surface ofsaid second diffuser, as a result of metal vapor condensation during thewelding operation, is reduced or prevented.